1. Field of the Invention
The invention relates to an electromagnetic actuator for locking or releasing relative rotation between relative rotation members and an electromagnetic clutch using the same, and, specifically, a differential using the electromagnetic clutch.
2. Description of Relevant Art
The patent application publication laid-open No.Hei11-186031 as a relevant document describes “a method of actuating a solenoid and an actuator”.
When a general electromagnetic actuator actuates, for example, an engagement clutch (clutch system), firstly a large solenoid current is supplied to an electromagnetic solenoid. This allows the electromagnet to be excited, thus engaging the engagement clutch. Thereafter, the solenoid current is reduced to a hold current that is sufficient to hold the clutch engaged.
The following is objectives for supplying the excitation current when the engagement clutch engages. The first objective is to provide a large actuation force to the engagement clutch for secure engagement without enlarging the electromagnetic solenoid. The second objective is, thereafter, to reduce the solenoid current to the hold current, thus reducing power consumption and heat generation.
According to the solenoid actuating method in the relevant document, based on the excitation current (Ia, Ib) measured at at least two time points (Ta, Tb) during the excitation, completion time points (Tg) are predicted that the excitation current reaches a predetermined value. At the predicted time point (Tg), the excitation current is switched to the hold current. This establishes timing to switch the solenoid current to the hold current.
Thus, according to the solenoid driving method in the relevant document with a system that predicts the completion time point when the excitation current reaches a predetermined value, if it takes time for the engagement clutch to practically engage more than predicted, the solenoid current is reduced to the hold current before the engagement clutch engages. This prediction causes a shortage of the actuation force, thus failing to engage the engagement clutch.
For preventing the shortage of the actuation force, however, the electromagnetic solenoid requires normal supplying of the excitation current, which increases current consumption and heat generation.
For normal supplying of the excitation current to the electromagnetic solenoid to reduce the solenoid current (or excitation current), the electromagnetic solenoid is required to be larger. This renders the system larger, and deteriorates the layout and vehicle mountability.